Since most electronic products need stable and non-noise DC electricity for providing power, the AC electricity provided by the electric power company must pass through a rectifier for converting the AC electricity into a first DC electricity. But, no mater utilizing half wave rectifier or full wave rectifier, the level of the converted first DC electricity still alters constantly just like the DC electricity, so that the electronic products can not utilize the unstable first DC electricity. Therefore, the converter becomes very important. Converters are used to convert the first DC electricity into a second DC electricity, which can have a fixed level for forming a stable output. Besides, the level of the second DC electricity also can be altered through setting the circuit of the converter. The architecture of the converter used currently is shown in FIG. 1. As shown, the converter includes a half bridge switch unit 21, an electricity converting unit 22, a PWM (Pulse Width Modulation) control unit 23 for producing a conduction cycle signal and a feedback unit 24 for producing a feedback signal. The half bridge switch unit 21 is connected to a DC power source 1 for obtaining a first DC electricity. The conduction cycle signal controls the half bridge switch unit 21 to conduct the duty cycle that the first DC electricity passes through the electricity converting unit 22. The electricity converting unit 22 converts the first DC electricity into a second DC electricity for outputting. The feedback unit 24 draws a feedback signal from the second DC electricity to send to the PWM control unit 23, so as to adjust the conduction cycle signal for altering the duty cycle of the half bridge switch unit 21, thereby achieving a feedback voltage regulation of the output of the converter which is modulated according to the load. In the above described architecture, if the half bridge switch unit is a half bridge switch circuit, for achieving the best working efficiency of the converter, the optimum condition is to let the half bridge switch circuit having a duty cycle ratio of 50:50, that is, the opening of the switch element in the half bridge switch circuit occupies 50% of the duty cycle. For preventing the output of the converter from alteration owing to the load, the output feedback can be modulated according to the alteration of the duty cycle ratio of the half bridge switch circuit. Therefore, since the converter can not maintain a 50:50 duty cycle ratio for providing the best efficiency all the time, the conversion efficacy will be lower than the optimum value. However, the reduction of efficacy namely represents the increment of loss, and the loss may become waste heat or may destroy the structure of electronic element so as to reduce the life time of the converter. Consequently, there is the need to provide an improvement.